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I am looking at the possibility of making a Q meter for the frequency range 1.5 MHz to 150 MHz. One common way to do this is to inject a voltage from a LOW impedance source, into a series tuned circuit, then measure the voltage developed across the tuned circuit with a high input impedance voltmeter. The Q is the ratio of those two voltages.?
Designs for Q meters usually include an oscillator, but I don’t think I will bother with that as I can easily use a signal generator.?
One design appeared in Practical Wireless in the November 1978 issue. I stuck a copy of that in a sub-directory for Q measurements in the files section of the forum?
/g/Test-Equipment-Design-Construction/files/Q-factor%20%28Q%20is%20the%20inverse%20of%20dissipation%20factor%20DF%29.
There are two problems I see with that design
1) The meter has measurement ranges of only 0-20 and 0-100, but many inductors have Q’s much higher than 100. The HP 4342A can measure Qs from 5 to 1000. The old Boonton 160-A measures Q up to 640 (from memory).?
2) The output impedance of source is too high?- it is approximately 2 ohm on the Q=0-20 range and 10.2?ohm on the Q=0-100 range.?
Can anyone think of a way of turning a signal generator to have an output impedance of 1 milli ohm? I think I have a 5 W Minicircuits amplifier around, so I can afford to waste a bit in heat. But it is not practical to put 50 ohm in series with 1 milli ohm to terminate the amplifier in the required 50 ohm. I will have so little voltage left, it will be difficult to measure the output voltage, even though it’s multipled by the Q of the coil. ?
A step-down transformer seems the most obvious way, but that requires a turns ratio of sqrt(50000)=224. Even with a single turn on the secondary, there will be too many turns on the primary for this to work at 150 MHz.
?
I have not yet looked at the HP 4343A meter service manual. That will probably give me some ideas.?
I suspect the answer is to not try to get such a low output impedance, accept that the voltage generated across the LC combination will be less than the Q, and correct for that in software.?
Any other thoughts??
PS, Does anyone have any documentation, apart from the user manual, on the HP 42851A Q-adapter, which is used with the HP/Agilent 4285A Precision LCR meter??
?
?
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Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket
has a
single turn loop attached with sufficient distance between the two
loops
to allow the coil under test to be positioned between them. one turn
is fed
from your sig' gen' and the other feeds a scope or RF meter.? A
sufficient
level from the sig gen may require an amplifier. With the coil to be
tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning
the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the
coil.
The coil being tested can, of course, have a capacitor in parallel
if wished.
73 Ken G8BEQ
?
On 04/08/2022 09:19, Dr. David Kirkby,
Kirkby Microwave Ltd wrote:
toggle quoted message
Show quoted text
I am looking at the possibility of making a Q meter for the
frequency range 1.5 MHz to 150 MHz. One common way to do this is
to inject a voltage from a LOW impedance source, into a series
tuned circuit, then measure the voltage developed across the
tuned circuit with a high input impedance voltmeter. The Q is
the ratio of those two voltages.?
Designs for Q meters usually include an oscillator, but I don’t
think I will bother with that as I can easily use a signal
generator.?
One design appeared in Practical Wireless in the November 1978
issue. I stuck a copy of that in a sub-directory for Q
measurements in the files section of the forum?
/g/Test-Equipment-Design-Construction/files/Q-factor%20%28Q%20is%20the%20inverse%20of%20dissipation%20factor%20DF%29.
There are two problems I see with that design
1) The meter has measurement
ranges of only 0-20 and 0-100, but many inductors have Q’s much
higher than 100. The HP 4342A can measure Qs from 5 to 1000. The
old Boonton 160-A measures Q up to 640 (from memory).?
2) The output impedance of source is too high?- it is
approximately 2 ohm on the Q=0-20 range and 10.2?ohm on the
Q=0-100 range.?
Can anyone think of a way of turning a signal generator to have
an output impedance of 1 milli ohm? I think I have a 5 W
Minicircuits amplifier around, so I can afford to waste a bit in
heat. But it is not practical to put 50 ohm in series with 1
milli ohm to terminate the amplifier in the required 50 ohm. I
will have so little voltage left, it will be difficult to
measure the output voltage, even though it’s multipled by the Q
of the coil. ?
A step-down transformer seems the most obvious way, but that
requires a turns ratio of sqrt(50000)=224. Even with a single
turn on the secondary, there will be too many turns on the
primary for this to work at 150 MHz.
?
I have not yet looked at the HP
4343A meter service manual. That will probably give me some
ideas.?
I suspect the answer
is to not try to get such a low output impedance, accept that
the voltage generated across the LC combination will be less
than the Q, and correct for that in software.?
Any other thoughts??
PS, Does anyone have any documentation, apart from the
user manual, on the HP 42851A Q-adapter, which is used with
the HP/Agilent 4285A Precision LCR meter??
?
?
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I see another problem that I was unaware of. There are two definitions of Q I am aware of?
1) Q = 2 P Energy_stored_per_cycle / Energy _dissipated_per_cycle
2) Q = fr (f2 - f1)
where fr is the resonant frequency and f1 and f2 the 3 dB points (voltage falls by 1/sqrt(2) )
Here’s a proof that the two definitions are equal?
Wikipedia says that they are only approximately equal, with them agreeing more at high Q. Wikipedia gives a link to an article that they are not equal, but it’s a book with text too small to read on my iPhone.
It seems that a Q-meter might need a switch where you select what definition you want.?????
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On Thu, Aug 4, 2022 at 02:14 AM, Kenneth Greenough wrote:
Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket has a
single turn loop attached with sufficient distance between the two loops
to allow the coil under test to be positioned between them. one turn is fed
from your sig' gen' and the other feeds a scope or RF meter.? A sufficient
level from the sig gen may require an amplifier. With the coil to be tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the coil.
The coil being tested can, of course, have a capacitor in parallel if wished.
73 Ken G8BEQ
I have seen something like that before. One problem is that it would not work with coils in screened enclosures, some of which are.?I have ordered a couple of Boonton Q-standards, which I was hoping to measure. Those are in screen enclosures, with 2 banana plugs 25.4 mm (1”) apart.?See attached photograph.?
I think capacitive coupling is also possible If the capacitors are very small, the reactance is high, so they present little loading to the coil/capacitor combination.
One really does need a capacitor in parallel with the coil so that one can make the measurement at the frequency of interest, rather than the self resonant frequency of the inductor.?
Dave
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I think the best method for measuring loaded and unloaded Q and at the same time the coupling factor
is by using the resonance circle that is created on the smith chart of VNA. Read the article by late Darko Kajfez from Miss State Univ (olemiss)
it becomes a bit tricky for very high unloaded Q or very weak couplings but still usable and I had seen several imporvements
on that method but i cannot remember where...I have only used his method with a simple computer program that I wrote
to analyze the S11 collected from the VNA
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If you have not already done so, you may want to read through the Boonton 260 Q Meter manual.
It is a wealth of information and provides a lot of insight on where errors creep into measurements.
The manual can be found at the following link:?
Boonton's approach was to use a 20 milliOhm precision resistor with an attached thermocouple that
drove a panel meter. By measuring the power dissipation of the resistor, they obtained a known
voltage drop across the resistor. The integral VTVM measured the voltage across the tuned circuit
and the display meter was scaled in Q based on the results of the ratio of the tuned circuit voltage
to the voltage across the precision resistor.
HP bought out Boonton and marketed the 260A for a few years before presenting the HP4342 series
of Q Meters. I have not confirmed but was told that HP abandoned use of the precision resistor in
the 4342A. I used the 4342A for years in the 80's through the early 2000's. It is a tough box and in
my opinion, if you can buy one with unmolested innards, it is well worth the cost of repair and calibration
in a metrology lab.
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Have a look at the manual of the Rhode & Schwartz? QMeter
On Thursday, August 4, 2022 at 11:55:19 AM GMT+2, Dr. David Kirkby, Kirkby Microwave Ltd <drkirkby@...> wrote:
On Thu, Aug 4, 2022 at 02:14 AM, Kenneth Greenough wrote:
Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket has a
single turn loop attached with sufficient distance between the two loops
to allow the coil under test to be positioned between them. one turn is fed
from your sig' gen' and the other feeds a scope or RF meter.? A sufficient
level from the sig gen may require an amplifier. With the coil to be tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the coil.
The coil being tested can, of course, have a capacitor in parallel if wished.
73 Ken G8BEQ
I have seen something like that before. One problem is that it would not work with coils in screened enclosures, some of which are.?I have ordered a couple of Boonton Q-standards, which I was hoping to measure. Those are in screen enclosures, with 2 banana plugs 25.4 mm (1”) apart.?See attached photograph.?
I think capacitive coupling is also possible If the capacitors are very small, the reactance is high, so they present little loading to the coil/capacitor combination.
One really does need a capacitor in parallel with the coil so that one can make the measurement at the frequency of interest, rather than the self resonant frequency of the inductor.?
Dave
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Have a look at the manual of the Rhode & Schwartz? QMeter
On Thursday, August 4, 2022 at 11:55:19 AM GMT+2, Dr. David Kirkby, Kirkby Microwave Ltd < drkirkby@...> wrote:
On Thu, Aug 4, 2022 at 02:14 AM, Kenneth Greenough wrote:
Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket has a
single turn loop attached with sufficient distance between the two loops
to allow the coil under test to be positioned between them. one turn is fed
from your sig' gen' and the other feeds a scope or RF meter.? A sufficient
level from the sig gen may require an amplifier. With the coil to be tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the coil.
The coil being tested can, of course, have a capacitor in parallel if wished.
73 Ken G8BEQ
I have seen something like that before. One problem is that it would not work with coils in screened enclosures, some of which are.?I have ordered a couple of Boonton Q-standards, which I was hoping to measure. Those are in screen enclosures, with 2 banana plugs 25.4 mm (1”) apart.?See attached photograph.?
I think capacitive coupling is also possible If the capacitors are very small, the reactance is high, so they present little loading to the coil/capacitor combination.
One really does need a capacitor in parallel with the coil so that one can make the measurement at the frequency of interest, rather than the self resonant frequency of the inductor.?
Dave
-- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@...Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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On? Thu, 4 Aug 2022 9:19? Dr. David Kirkby wrote:
"A step-down transformer seems the most obvious way...? I have not yet looked at the HP 4343A meter service manual. That will probably give me some ideas. "
------------------------------------------------------------
The
HP4342A Service manual is vague about the
injection transformer, but it is driven by a cascaded emitter follower.?
There is a 75 Ohm resistor across the primary winding to stabilise the
driver load impedance outside DUT resonance.
HP
Journal September 1970
(at https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1970-09.pdf) is more
forthcoming and has some other interesting details about the HP4342A design.?
I do wonder what ferrite was used and how
they wound the toroidal transformer to avoid self resonance and obtain "a nearly flat frequency response" over the design bandwidth 22kHz to 70MHz.? The turns ratio was 50:1.
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I have a real Boonton Q meter.? Yes it was made with tubes (valves).? But it was a truly outstanding meter in its day.? David, I would consider giving it to you for trade for something else small but getting it to you is a a problem. I think it would be cheaper for me to fly there with it in a suitcase than to ship it.
73,
Jeffrey Pawlan? WA6KBL
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If you have not already done so, you may want to read through the Boonton 260 Q Meter manual.
It is a wealth of information and provides a lot of insight on where errors creep into measurements.
The manual can be found at the following link:?
Thank you. That is the best 260-A manual I have seen, although I have seen a “manual” that covered a few of the Boonton models. I always felt that there should be one specific to each model.
Reading that, I do get the feeling that some of the improvements made between the 160-A and 260-A, would not be necessary if an instrument was made with a microprocessor doing some number?crunching. For example, they reduced the injection resistor from 40 milli ohm to 20 milli ohm. But still it’s a source of error at high Q which one can correct with the aid of a calculation. If an instrument was made nowadays, one might just as well correct every reading ?Maybe the resistors used in the PW design would be okay if a microprocessor did the corrections, rather than?
Boonton's approach was to use a 20 milliOhm precision resistor with an attached thermocouple that
drove a panel meter. By measuring the power dissipation of the resistor, they obtained a known
voltage drop across the resistor. The integral VTVM measured the voltage across the tuned circuit
and the display meter was scaled in Q based on the results of the ratio of the tuned circuit voltage
to the voltage across the precision resistor.
I don’t understand why those thermocouples are so easily damaged.?
HP bought out Boonton and marketed the 260A for a few years before presenting the HP4342 series
of Q Meters. I have not confirmed but was told that HP abandoned use of the precision resistor in
the 4342A. I used the 4342A for years in the 80's through the early 2000's. It is a tough box and in
my opinion, if you can buy one with unmolested innards, it is well worth the cost of repair and calibration
in a metrology lab.
_._,_._,_
If I bought one I would send it to Keysight - assuming that they will still calibrate it, which I suspect that they will. I have a healthy scepticism of calibration labs.?
Dave -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@...Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Have a look at the manual of the Rhode & Schwartz? QMeter
Do you know of a model number(s)? My search with Google didn’t bring up anything useful. There are plenty of LCR meters, but unless significantly different from my 4284A and 4285A, they are pretty poor at measuring Q above about 20.?
Dave?
On Thursday, August 4, 2022 at 11:55:19 AM GMT+2, Dr. David Kirkby, Kirkby Microwave Ltd < drkirkby@...> wrote:
On Thu, Aug 4, 2022 at 02:14 AM, Kenneth Greenough wrote:
Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket has a
single turn loop attached with sufficient distance between the two loops
to allow the coil under test to be positioned between them. one turn is fed
from your sig' gen' and the other feeds a scope or RF meter.? A sufficient
level from the sig gen may require an amplifier. With the coil to be tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the coil.
The coil being tested can, of course, have a capacitor in parallel if wished.
73 Ken G8BEQ
I have seen something like that before. One problem is that it would not work with coils in screened enclosures, some of which are.?I have ordered a couple of Boonton Q-standards, which I was hoping to measure. Those are in screen enclosures, with 2 banana plugs 25.4 mm (1”) apart.?See attached photograph.?
I think capacitive coupling is also possible If the capacitors are very small, the reactance is high, so they present little loading to the coil/capacitor combination.
One really does need a capacitor in parallel with the coil so that one can make the measurement at the frequency of interest, rather than the self resonant frequency of the inductor.?
Dave
-- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@...Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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On Thu, 4 Aug 2022 at 14:34, Jeffrey Pawlan < pawlan@...> wrote: I have a real Boonton Q meter.? Yes it was made with tubes (valves).? But it was a truly outstanding meter in its day.? David, I would consider giving it to you for trade for something else small but getting it to you is a a problem. I think it would be cheaper for me to fly there with it in a suitcase than to ship it.
73,
Jeffrey Pawlan? WA6KBL
Well Jeffrey, you bring up an interesting point. I bought a 160-A from the USA a few days ago.
The shipping was a suspiciously low $21.72.? It's going via the eBay Global Shipping Program which is run by Pitney Bowes. The tracking shows it was picked up in California, and is still in
California, but is due for delivery tomorrow by FedEx in Kentucky, which I assume is the Global Shipping Centre. Now what happens there I do not know. If they realise they are only being paid $21.72 to ship it to the UK, they might refuse to ship it. The other possibility, and one I fear, is they will ship it with inadequate carriage, and I will be expected to pay for the excess carriage charge. In which case I will decline. If I'm going to have to pay $150-$200 in shipping charges, it is not worth my while. The meter cost me $75.
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You raise the issue here of how to generate a signal with very low source resistance.
How about this idea.
Consider a signal source feeding a resistive divider consisting of Rs (source) and Rl (load).? This gives the relationship? Rs/Rl = Vs/Vl-1. So Rs approaches 0 as Vs and Vl become equal.
If you introduce an ALC control loop that monitors Vl and attempts to make Vs and Vl equal then the effective Rs becomes small.
A worthy idea or not ?
I played with this a bit recently and could quite easily get Rs down to one ohm at 50MHz.? Its part of an idea to modernize the old Boonton 250 RX meter.?
Bill
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On Thu, 4 Aug 2022 at 14:00, Navmar < navmar@...> wrote: You raise the issue here of how to generate a signal with very low source resistance.
How about this idea.
Consider a signal source feeding a resistive divider consisting of Rs (source) and Rl (load).? This gives the relationship? Rs/Rl = Vs/Vl-1. So Rs approaches 0 as Vs and Vl become equal.
If you introduce an ALC control loop that monitors Vl and attempts to make Vs and Vl equal then the effective Rs becomes small.
A worthy idea or not ?
I played with this a bit recently and could quite easily get Rs down to one ohm at 50MHz.? Its part of an idea to modernize the old Boonton 250 RX meter.?
Bill
Could you please sketch it out, as I am not following it.
Dave
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On 8/4/2022 3:19 AM, Dr. David Kirkby,
Kirkby Microwave Ltd wrote:
Can anyone
think of a way of turning a signal generator to have an output
impedance of 1 milli ohm?
Like most engineering
decisions, it depends.? In this case my first though would be
negative feedback around an amp.? You can get almost
arbitrary reduction of output impedance as a tradeoff for other
parameters.? This is much more practical at much higher
frequencies than years past.
At frequencies where? negative feedback is not practical,
conventional transformers are not terribly likely to useful or
practical.? It may be that the kind of transformers used in RF
power amplifiers may be as they can be matching sub one ohm
impedances to 50 ohms.
Finally, filter structures can achieve wide impedance
transformations.? Of couirse these are subject the practical
limitations of components.? Since you are focusing on output
impedance you are likely to make different optimizations are so
find the impedance transformations you wish more achievable.
Oz (in DFW, Texas)
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I also have Boonton 260A. I
had series of 5 coils that had Qs over 1000. I wanted to see
about coil spacing, On these 6" coils I found 10 to 12 turns per
inch got the highest Q using 660/43 Litz wire. Graph of coil Qs,
?I used a BNC T to tap into the RF drive and connected an RF
voltmeter to measure the drive, RF drive needs to be lowered
below the reading of the existing Multiply by meter. Then you
have a new multiplier times the Q meter reading. New multiplier
number depends on drive voltage.
?I found good agreement with the 3db method, but the 3db method
gets tricky at Qs over 1000!
You probably know but the 260a manual is here, good theory
section also.
??????????????????????? Mikek
?
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This site has a novel way to
Counter inductance of the 0.02Ω output resistor, but
he still only gets to 40MHz, But maybe you could have two
transformers, one for above 40MHz and one for below.
At bottom of page.
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Here is the Q page of W7ZOI
with his Q meter method and lots of theory.
??????????????????? Mikek
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Hello everyone,
You may want to have a look at this article that I wrote in QEX, ten years ago:
Q Factor Measurements on L-C Circuits.
Jacques,? VE2AZX
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Dr. David Kirkby, Kirkby Microwave Ltd
